Frenchman Gael Monfils, seeded 13th, became the first seeded victim, going out in a barrage of 21 aces by Croatian Ivo Karlovic 6-4, 7-6 (7-5), 7-6 (7-2). Spain's 15th seed Tommy Robredo lost to Jeremy Chardy of France 6-3, 7-5.

There were seeded wins for French player Gilles Simon as the number nine put out Wayne Odesnik 6-3, 6-2. Number 14 Croatian Marin Cilic beat Spanish veteran Juan Carlos Ferrero 6-3, 6-4.

Unseeded Swiss Stan Wawrinka lost to Spaniard David Ferrer 7-5, 6-2, while Russian Igor Andreev handed Nicolas Kiefer a second consecutive first-round Masters loss, 6-1, 7-5.

Spain's Rafael Nadal, displaced from second to third on the ATP by Murray's rise, said he's not stressed about losing the ranking or his comeback after two months out due to damaged knees.

'It's impossible start from a zero to 100,' said Nadal, quarter-finalist last week in Montreal as he played his first tournament since the French Open.

'I need to progress every day a little bit more. Today I trained for one hour, 45 minutes, instead of two and a half hours. The important thing is be good physically. I know if I work hard and if I can work with the normal intensity like in the past, I'll have chances to return to my best level. But I have to stay calm.'